An understanding of such processes as secretion, fertilization, and myogenesis is dependent upon an understanding of how membranes fuse. The research proposed here is designed to analyze sexual fusion between Chlamydomonas gametes, a process that occurs at discrete, highly differentiated sites within the cell membrane and normally proceeds with 100% efficiency. To dissect which architectural features of these sites are critical for fusion, mutant strains that cannot fuse will be examined by thin-section and freeze-fracture electron microscopy. The effect of temperature and ionic environment on the fusion process will also be analyzed, and the ability of fusion-primed cells to fuse with heterologous substrates (e.g., liposomes) will be explored. The project should elucidate which, if any, of the various "fusogenic" protocols defined by in vitro experiments are operative in a well-defined in vivo fusion event.